1. Field of the Invention
The present invention relates generally to non-linear optical systems, and in particular to improved non-linear optical systems and techniques employing higher-order mode optical fibers.
2. Background Art
In a non-linear optical system; such as a non-linear microscopy system, a pulsed laser beam is tightly focused onto a sample, causing an optical output to be generated therefrom. A non-linear signal can be derived from the optical output, and this non-linear signal can be used to generate a microscopic image of the sample. A number of different higher-order light-matter interactions may be used in a non-linear optical system, including two-photon fluorescence, second-harmonic generation, third-harmonic generation, Raman scattering, and the like. In a multiphoton emission process, the relationship between incident light intensity and emitted radiation is nonlinear. For example, for two-photon excitation, the relationship is quadratic. As a result of this nonlinear relationship, only the central spatial portion of a conventional Gaussian beam substantially contributes to the intensity of emitted radiation. Therefore, much more multiphoton radiation is generated where the laser beam is tightly focused than where it is more diffuse. Effectively, excitation is restricted to the focal volume, resulting in a high degree of rejection of out-of-focus objects Similar effects occur in other types of light-matter interactions, including second-harmonic generation, third-harmonic generation, Raman scattering, optically-induced chemical reactions, material breakdown and the like.
FIG. 1A is a diagram illustrating the basic principles of operation of an exemplary non-linear microscopy system 20 according to the prior art. A femtosecond laser 22 provides incident light in the form of a pulsed laser output that is guided by a single-mode fiber (SMF) 24 having an end face 26 that provides the pulsed laser beam 28 as a free-space output. The laser beam 28 is directed by a scanning mirror assembly 30 to an objective lens 32 that focuses the beam onto a sample 34 for which a microscopic image is to be generated. The laser beam 28 has an intensity that is sufficient to cause multiphoton excitation of fluorophores in an excitation volume of the sample. Fluorescence 36 is emitted having an intensity level indicating the amount of multiphoton excitation for example two-photon fluorescence.
The number of photons required for excitation depends upon the particular type of light-matter interaction used to create fluorescence. In the present discussion, microscopy system 20 is assumed to use two-photon excitation. However, it will be appreciated that the present discussion applies to non-linear microscopy employing other types of light-matter interactions, including second-harmonic generation, third-harmonic generation, Raman scattering, and the like.
The emitted fluorescence is detected by a suitable detector 38, such as a photodiode, photomultiplier, or like device. Scanning the focused laser beam 28 over a region of the sample allows point-by-point intensity data to be gathered. Alternatively, the position of the beam could be kept fixed and the sample scanned in position with respect to the beam. An image generator 40 then uses the intensity data to generate a microscopic image of the scanned sample region.
FIG. 1B shows a diagram of a scanning confocal non-linear microscopy system 50 according to the prior art. Like the microscopy system shown in FIG. 1A, system 50 includes a femtosecond laser 52 and a single mode fiber 54 having an end face 56 that provides a pulsed laser output 58. A scanning mirror assembly 60 directs the laser output 58 to an objective lens 62 that tightly focuses it onto a sample 64, thereby resulting in multiphoton excitation of fluorophores in an excitation volume and the emission of fluorescence 66.
The FIG. 1B microscope includes an output pinhole assembly 68, which is provided by a using a detector lens 70 to focus the emitted fluorescence and provide it as an input into a second single-mode fiber (SMF) 72, which in turn guides the focused fluorescence to a detector 74, which provides fluorescence intensity data to image generator 76. The use of pinhole assembly 68 adds depth (i.e., axial) resolution to the microscopy system 50, as only signals generated in the focus of the incident light beam can efficiently couple into the second SMF 70 for measurement by the detector 72. With this setup, a three-dimensional non-linear image of a sample can be obtained by scanning the sample transversely, as well as axially.
In the microscopy systems shown in FIGS. 1A and 1B, the pulsed laser beam used to provide incident light to the sample is guided from the laser to the objective lens using the fundamental LP01 transverse mode, which has a near-Gaussian shape. In both systems 20, 50, the single-mode fiber 24, 54 that guides the laser beam to the objective lens does not support propagation in higher-order modes, which have distinctly non-Gaussian shapes.
Because of its near-Gaussian shape, an LP01 mode laser beam can be focused to a narrower beam width than a higher-order mode laser beam. For this reason, current non-linear microscopy systems have used an LP01 mode laser beam to provide incident excitation light. The localization of excitation in a non-linear optical system typically results in significantly higher spatial resolution than that achievable in a linear optical system. However, it would be desirable to improve the performance of microscopy systems even further. In particular, it would be desirable to find ways to enhance the signal resolution in multiple dimensions.